1. Hokkaido Institute of Public Health; Sapporo Hokkaido 060 Japan
2. Department of Bacteriology; Okayama University Medical School; Okayama Okayama 700 Japan
3. Department of Veterinary Microbiology; Obihiro University of Agriculture and Veterinary Medicine; Obihiro Hokkaido 080 Japan
4. Department of Preventive Dentistry; Health Sciences University of Hokkaido; Ishikari-Tobetsu Hokkaido 061-02 Japan
5. Department of Microbiology, School of Medicine; University of Occupational and Environmental Health; Kitakyusyu Fukuoka 807 Japan